A differential circuit has a good immunization against electromagnetic interference, power supply noise and ground noise and has a good performance in suppressing even harmonic, therefore, a circuit of a differential structure becomes more and more popular in a radio-frequency circuit, such as a low-noise amplifier, a frequency mixer and a power amplifier. In an RF communication system, signals sent and received by an antenna are single-ended signals, so that a transformer or a transformer-type balun (balanced to unbalanced), as a module for conversion between a single-end signal and a differential signal, becomes essential in an existing RF circuit system. In a receiver, a transformer converts a single-end signal received by an antenna into a differential signal, and then transmits the differential signal to a low-noise amplifier of a differential structure for amplification; and in a transmitter, a transformer converts a differential signal output by a power amplifier of a differential structure into a single-end signal and then transmits the single-end signal to an antenna for transmission. In consideration of the cost and area of a chip and other aspects, the transformer is usually integrated on a same chip with a receiving/transmitting module.
One of the most important performances of an on-chip transformer, which serves as a passive power transmission module, is insertion loss. When the transformer is located at the front end of a low-noise amplifier of a receiver as a conversion module for a single-end signal and a differential signal, the insertion loss of the transformer is equal to a noise coefficient. If the insertion loss of the transformer is too much, the sensitivity of the whole receiver may be reduced greatly. When the transformer is located at the front end of the transmitter as a conversion module for a differential signal and a single-end signal, the insertion loss of the transformer is equal to the power loss of a transmitted signal. To meet the power requirement of the transmitted signal, the insertion loss is as small as possible.
The insertion loss of the transformer mainly depends on three factors: a coupling coefficient between coils, a quality factor (Q value) of a coil inductor and an input-output return loss, wherein the coupling coefficient can be improved greatly by optimizing the winding way of a transformer coil. For example, the coupling coefficient can be above 0.9 if a laminated structure is adopted; and the input-output return loss can be improved by a tuning capacitor and optimizing the turns ratio of the coil. Under the current process condition, the quality factor of the coil inductor causes the severest impact on the insertion loss of the transformer.
The quality factor of an on-chip coil inductor may be affected by a plurality of factors, one of which is the eddy current loss of the inductor. An eddy current is an induced ring current generated by a magnetic field of the inductor on a conductor, such as the induced ring current on a substrate, a metal wire and a component. Generally speaking, the distance between the conductor and the transformer coil is smaller, the loss of the induced eddy current is more serious. At present, to reduce the influence of the conductor on the self-inductance quality factor of the transformer coil, most of the metal wires and components on the chip keep far away from the coil; thus, the transformer and its surrounding blank area occupy a large area of the chip, and the utilization ratio of the area of the whole chip is very low.
A multi-mode multi-band system is the development trend of the current mobile communication system, transmitting and receiving modules in different frequency bands share one or more antennas, and the transmitting and receiving modules in each frequency band need correspond to one transformer. In consideration of the existing process condition, the cost of the chip and other aspects, the area of the on-chip transformer should not be too large and is usually below 400 um*400 um, so that the self-inductance value of the transformer coil is very small. When the transformer works in a low frequency band range, such as in a frequency band of hundreds of megahertz, its input and output ports need to be connected with a large tuning capacitor in parallel to enable the whole transformer to resonate in a low-frequency working frequency band.
In addition, if the receiving or transmitting module of each frequency band corresponds to one transformer-type balun, the area of the whole chip is very large, accordingly, the cost of the chip is increased greatly. At present, to reduce the number of the transformers on the chip, a common method is to connect a tuning capacitor with the input end and the output end of the transformer in parallel respectively, and a same transformer can work in different frequency bands by changing the value of the tuning capacitor, so as to implement the multiplexing of the same transformer in different frequency bands.
On the whole, the transformer and the tuning capacitor in the multi-mode multi-band system occupy a very large area of the chip, and the Q value of the coil inductor of the transformer is small, so, some measures are needed to be taken to reduce the area of the chip that the transformer and its tuning capacitor occupy and improve the Q value of the coil inductor of the transformer.